package lan.internal.loader;

import java.io.IOException;
import java.net.URL;
import java.nio.file.InvalidPathException;
import java.nio.file.Path;
import java.security.CodeSource;
import java.security.PermissionCollection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.jar.Manifest;

import lan.internal.access.JavaLangAccess;
import lan.internal.access.SharedSecrets;
import lan.internal.misc.VM;
import lan.reflect.generics.reflectiveObjects.NotImplementedException;

/**
 * Creates and provides access to the built-in platform and application class
 * loaders. It also creates the class loader that is used to locate resources
 * in modules defined to the boot class loader.
 */

public class ClassLoaders {

    private ClassLoaders() { }

    private static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();

    // the built-in class loaders
    private static final BootClassLoader BOOT_LOADER;
    private static final PlatformClassLoader PLATFORM_LOADER;
    private static final AppClassLoader APP_LOADER;

    // Creates the built-in class loaders.
    static {
        // -Xbootclasspath/a or -javaagent with Boot-Class-Path attribute
        String append = VM.getSavedProperty("jdk.boot.class.path.append");
        BOOT_LOADER =
                new BootClassLoader(append, true);
        PLATFORM_LOADER = new PlatformClassLoader(BOOT_LOADER);

        // A class path is required when no initial module is specified.
        // In this case the class path defaults to "", meaning the current
        // working directory.  When an initial module is specified, on the
        // contrary, we drop this historic interpretation of the empty
        // string and instead treat it as unspecified.
        String cp = System.getProperty("java.class.path");
        if (cp == null || cp.isEmpty()) {
            String initialModuleName = System.getProperty("jdk.module.main");
            cp = (initialModuleName == null) ? "" : null;
        }
        //URLClassPath ucp = new URLClassPath(cp, false);
        APP_LOADER = new AppClassLoader(PLATFORM_LOADER, cp, false);
    }

    /**
     * Returns the class loader that is used to find resources in modules
     * defined to the boot class loader.
     *
     * @apiNote This method is not public, it should instead be used via
     * the BootLoader class that provides a restricted API to this class
     * loader.
     */
    static BuiltinClassLoader bootLoader() {
        return BOOT_LOADER;
    }

    /**
     * Returns the platform class loader.
     */
    public static ClassLoader platformClassLoader() {
        return PLATFORM_LOADER;
    }

    /**
     * Returns the application class loader.
     */
    public static ClassLoader appClassLoader() {
        return APP_LOADER;
    }

    /**
     * The class loader that is used to find resources in modules defined to
     * the boot class loader. It is not used for class loading.
     */
    private static class BootClassLoader extends BuiltinClassLoader {
        BootClassLoader(String path, boolean skipEmptyElements) {
            super(null, null, path, skipEmptyElements);
        }

        @Override
        public Class<?> loadClassOrNull(String cn, boolean resolve) {
            return JLA.findBootstrapClassOrNull(this, cn);
        }

    };

    /**
     * The platform class loader, a unique type to make it easier to distinguish
     * from the application class loader.
     */
    private static class PlatformClassLoader extends BuiltinClassLoader {
        static {
            if (!ClassLoader.registerAsParallelCapable())
                throw new InternalError();
        }

        PlatformClassLoader(BootClassLoader parent) {
            super("platform", parent, null, true);
        }

        /**
         * Called by the VM to support define package for AppCDS.
         *
         * Shared classes are returned in ClassLoader::findLoadedClass
         * that bypass the defineClass call.
         */
        private Package definePackage(String pn, Module module) {
            return JLA.definePackage(this, pn, module);
        }
    }

    /**
     * The application class loader that is a {@code BuiltinClassLoader} with
     * customizations to be compatible with long standing behavior.
     */
    private static class AppClassLoader extends BuiltinClassLoader {
        static {
            if (!ClassLoader.registerAsParallelCapable())
                throw new InternalError();
        }

        final URLClassPath ucp;

        AppClassLoader(PlatformClassLoader parent, String path, boolean skipEmptyElements) {
            super("app", parent, path, skipEmptyElements);
            this.ucp = new URLClassPath(path, false);
        }

        @Override
        protected Class<?> loadClass(String cn, boolean resolve)
                throws ClassNotFoundException
        {
            // for compatibility reasons, say where restricted package list has
            // been updated to list API packages in the unnamed module.
            SecurityManager sm = System.getSecurityManager();
            if (sm != null) {
                int i = cn.lastIndexOf('.');
                if (i != -1) {
                    sm.checkPackageAccess(cn.substring(0, i));
                }
            }

            return super.loadClass(cn, resolve);
        }

        @Override
        protected PermissionCollection getPermissions(CodeSource cs) {
            PermissionCollection perms = super.getPermissions(cs);
            perms.add(new RuntimePermission("exitVM"));
            return perms;
        }

        /*
         * Called by the VM to support dynamic additions to the class path
         *
         * @see java.lang.instrument.Instrumentation#appendToSystemClassLoaderSearch
         */
        void appendToClassPathForInstrumentation(String path) {
            ucp.addFile(path);
        }

        /**
         * Called by the VM to support define package for AppCDS
         *
         * Shared classes are returned in ClassLoader::findLoadedClass
         * that bypass the defineClass call.
         */
        private Package definePackage(String pn, Module module) {
            return JLA.definePackage(this, pn, module);
        }

        /**
         * Called by the VM to support define package for AppCDS
         */
        public Package defineOrCheckPackage(String pn, Manifest man, URL url) {
            return super.defineOrCheckPackage(pn, man, url);
        }
    }

    /**
     * Attempts to convert the given string to a file URL.
     *
     * @apiNote This is called by the VM
     */
    @Deprecated
    private static URL toFileURL(String s) {
        try {
            // Use an intermediate File object to construct a URI/URL without
            // authority component as URLClassPath can't handle URLs with a UNC
            // server name in the authority component.
            return Path.of(s).toRealPath().toFile().toURI().toURL();
        } catch (InvalidPathException | IOException ignore) {
            // malformed path string or class path element does not exist
            return null;
        }
    }
}
